Method for Etching a Group III Nitride Semiconductor, Method for Producing a Group III Nitride Semiconductor Crystal, and Method for Producing a GaN Substrate

ABSTRACT

A mask layer is formed on a Ga polarity surface of the GaN substrate as a growth substrate. Subsequently, a protective film PF is formed on a N polarity surface of the GaN substrate. Then, a plurality of concave portions is formed from the mask layer extending to the GaN substrate, to thereby form a seed crystal. The seed crystal is etched in a Na melt, and a plurality of concave portions having a facet plane exposed. The seed crystal and the raw materials are placed in a crucible, and the pressure and temperature inside the crucible are increased. Thus, a target GaN layer is grown in the upward direction on the surface of the mask layer and the lateral direction over the concave portions.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method for etching a Group IIInitride semiconductor, to a method for producing a Group III nitridesemiconductor crystal and to a method for producing a GaN substrate.

2. Background Art

A variety of methods for producing a semiconductor crystal are known,and examples thereof include vapor phase growth methods such asmetalorganic chemical vapor deposition (MOCVD) and hydride vapor phaseepitaxy (HVPE), molecular beam epitaxy (MBE), and liquid phase epitaxy(LPE). One technique of LPE is a flux method employing a Na flux.

In the flux method, a molten mixture of metallic Na (sodium) andmetallic Ga (gallium) is reacted with nitrogen under pressure for thegrowth of a GaN crystal. This method is expected to produce aninexpensive and high-quality GaN substrate because GaN crystal can begrown at a comparatively low temperature (up to 1,000° C.) and lowpressure (up to 10 MPa).

In the case where a GaN crystal is grown on an underlayer (GaN or AlN)serving as a seed crystal through a flux method, the crystal propertiesof the GaN crystal are inherited from those of the underlayer. That is,the dislocation density of the semiconductor crystal to be grown isinherited from that of the underlayer. Therefore, the dislocationdensity of the grown semiconductor crystal is about 5×10⁶/cm² to1×10⁷/cm² as same order as that of the underlayer.

A smaller dislocation density for the grown semiconductor crystal ispreferred. For example, a dislocation density of 1×10⁵/cm² or less ispreferred. Thus, in order to produce a GaN crystal having a smallerdislocation density, the dislocation density must be considerablyreduced during the growth of a GaN crystal. Japanese Patent ApplicationLaid-Open (kokai) No. 2005-12171 discloses an example of such methodthat a mask layer is formed on a seed crystal and GaN is laterally grownover the mask.

Japanese Patent Application Laid-Open (kokai) No. 2006-131454 (paragraph[0010]) discloses a technique for slightly growing a semiconductorcrystal with the flux temperature kept at a lower temperature than thegrowth temperature of semiconductor crystal to be grown.

Japanese Patent Application Laid-Open (kokai) No. 2008-150239 disclosesa technique for using GaN on a sapphire substrate as a template, forminga protective film on a rear surface of the sapphire substrate, andgrowing GaN through a flux method.

However, in the above Japanese Patent Application Laid-Open (kokai) Nos.2005-12171, 2006-131454, and 2008-150239, the dislocation density of theGaN crystal is about 1×10⁶/cm². Thus, in order to produce a GaN crystalhaving a smaller dislocation density, the dislocation density must beconsiderably reduced during the growth of a GaN crystal.

SUMMARY OF THE INVENTION

The present invention has been conceived in order to overcome theaforementioned drawbacks involved in conventional techniques. Thus, anobject of the present invention is to provide an etching method havinggood reproducibility for the entire GaN growth surface, a method forproducing a Group III nitride semiconductor crystal having excellentcrystallinity, and a method for producing a GaN substrate.

In a first aspect of the present invention, there is provided a methodfor etching a Group III nitride semiconductor, the method comprising:

a protective film formation step of forming a protective film on a firstsurface being a N polarity surface of Group III nitride semiconductor;and

an etching step of etching at least a portion of a second surface ofGroup III nitride semiconductor in a melt containing at least Na.

In the above method for etching a Group III nitride semiconductor, thesecond surface of Group III nitride semiconductor is etched in a meltcontaining Na in a crucible. Etching proceeds as the temperatureincreases. However, the first surface is not etched since a protectivefilm is formed on the first surface of Group III nitride semiconductor.That is, etching of the second surface is promoted by the formation ofthe protective film. Here, when the Group III nitride semiconductor isGaN, the second surface is a Ga polarity surface.

A second aspect of the present invention is a specific embodiment of themethod for etching a Group III nitride semiconductor of the firstaspect, wherein, the etching method comprises a mask layer formationstep of forming a mask layer made of Al_(X)In_(Y)Ga_((1-X-Y))N (0≦X,0≦Y, X+Y≦1) on a portion of the second layer of the Group III nitridesemiconductor, and in the etching step, the second surface of the GroupIII nitride semiconductor is etched after the mask layer formation step.

A third aspect of the present invention is a specific embodiment of themethod for etching a Group III nitride semiconductor of the secondaspect, wherein the mask layer is formed of AlGaN.

A fourth aspect of the present invention is a specific embodiment of themethod for etching a Group III nitride semiconductor of any of the firstto third aspects, wherein the protective film is formed of at least oneselected from a group consisting of Al₂O₃, ZrO₂, and TiO₂.

A fifth aspect of the present invention is a specific embodiment of themethod for etching a Group III nitride semiconductor of any of the firstto fourth aspects, wherein, in the etching step, the temperature of themelt containing at least Na is 600° C. to 1,000° C.

A sixth aspect of the present invention is a specific embodiment of themethod for etching a Group III nitride semiconductor of any of the firstto fifth aspects, wherein, in the etching step, the concentration ofGroup III metal in the melt containing at least Na is 0 mol % to 5 mol%.

In a seventh aspect of the present invention, there is provided a methodfor producing a Group III nitride semiconductor crystal, the methodcomprising:

a protective film formation step of forming a protective film on a firstsurface being a N polarity surface of Group III nitride semiconductor;

an etching step of etching at least a portion of a second surface ofGroup III nitride semiconductor in a melt containing at least Na, tothereby form a seed crystal; and

a semiconductor crystal formation step of growing a target layer ofGroup III nitride semiconductor crystal on the seed crystal in a moltenmixture containing at least Group III metal and Na.

An eighth aspect of the present invention is a specific embodiment ofthe method for producing a Group III nitride semiconductor crystal ofthe seventh aspect, wherein, the method further comprises a mask layerformation step of partially forming a mask layer made ofAl_(X)In_(Y)Ga_((1-X-Y))N (0≦X, 0≦Y, X+Y≦1) on a second surface of GroupIII nitride semiconductor, to thereby form an unexposed portion of thesecond surface which is covered with the mask layer and the remainingportion of the second surface which is not covered with the mask layer,and in the etching step, the exposed portion of the second surface ofthe Group III nitride semiconductor is etched after the mask layerformation step.

A ninth aspect of the present invention is a specific embodiment of themethod for producing a Group III nitride semiconductor crystal of theeighth aspect, wherein, the mask layer formation step further comprisesa mask layer growth step of growing a uniform mask layer so as to coverthe entire second surface of the underlayer formed of Group III nitridesemiconductor, and a concave portion formation step of forming aplurality of concave portions in the underlayer by removing an area ofthe mask layer to expose a part of the underlayer.

In the above method for producing a Group III nitride semiconductorcrystal, the first surface does not undergo melting back since theprotective film is formed on the first surface being a N polaritysurface. Instead, the second surface undergoes greater melting back.Thus, the concave portion has a sufficiently large depth. With thesurface of the mask layer as a starting point, Group III nitridesemiconductor crystal is formed. On the other hand, non-crystal portionscomposed of flux are formed on the concave portions. By virtue of suchnon-crystal portions, threading dislocations are not inherited from theunderlayer by the formed semiconductor crystal.

A tenth aspect of the present invention is a specific embodiment of themethod for producing a Group III nitride semiconductor crystal of theeighth or ninth aspect, wherein the mask layer is formed of AlGaN.

An eleventh aspect of the present invention is a specific embodiment ofthe method for producing a Group III nitride semiconductor crystal ofany of the seventh to tenth aspects, wherein the protective film isformed of at least one selected from a group consisting of Al₂O₃, ZrO₂,and TiO₂

A twelfth aspect of the present invention is a specific embodiment ofthe method for producing a Group III nitride semiconductor crystal ofany of the seventh to eleventh aspects, wherein, in the etching step, afacet plane of the underlayer is exposed.

A thirteenth aspect of the present invention is a specific embodiment ofthe method for producing a Group III nitride semiconductor crystal ofthe twelfth aspect, wherein, in the semiconductor crystal formationstep, Group III nitride semiconductor crystal is grown on the mask layersuch that the facet plane is not buried with the Group III nitridesemiconductor crystal.

A fourteenth aspect of the present invention is a specific embodiment ofthe method for producing a Group III nitride semiconductor crystal ofthe thirteenth aspect, wherein, in the semiconductor crystal formationstep, a non-crystal portion defined by the facet plane and the bottomsurface of the Group III nitride semiconductor crystal is formed.

A fifteenth aspect of the present invention is a specific embodiment ofthe method for producing a Group III nitride semiconductor crystal ofany of the seventh to fourteenth aspects, wherein the mask layer is aGroup III nitride semiconductor having an Al composition ratio of 0.02to 1.00 since the Group III nitride semiconductor layer containing Alwithin this range is virtually undissolved in the flux.

A sixteenth aspect of the present invention is a specific embodiment ofthe method for producing a Group III nitride semiconductor crystal ofany of the seventh to fifteenth aspects, wherein the underlayer is aGroup III nitride semiconductor having an Al composition ratio of 0 to0.02, and the Al composition ratio of the underlayer is smaller thanthat of the mask layer.

A seventeenth aspect of the present invention is a specific embodimentof the method for producing a Group III nitride semiconductor crystal ofany of the seventh to sixteenth aspects, wherein, in the etching step,the temperature of the melt containing at least Na is 600° C. to 1,000°C.

In an eighteenth aspect of the present invention, there is provided amethod for producing a GaN substrate, the method comprising:

a protective film of forming a protective film on a first surface beinga N polarity surface of Group III nitride semiconductor;

an etching step of etching at least a portion of a second surface ofGroup III nitride semiconductor in a melt containing at least Na, tothereby form a seed crystal;

a semiconductor crystal formation step of growing a GaN crystal on theseed crystal in a molten mixture containing at least Ga and Na; and

a semiconductor crystal separation step of separating the GaN crystalfrom the seed crystal.

The present invention can promote the etching of the second surface byforming the protective film on the first surface being a N polaritysurface of Group III nitride semiconductor. This enables stableformation of deep concave portions having a depth of tens to hundredsp.m. By using the seed crystal provided with deep concave portions, theinheritance of dislocations is blocked. Thus, a method for producing aGroup III nitride semiconductor crystal having excellent crystallinityand a method for producing a GaN substrate are achieved.

BRIEF DESCRIPTION OF THE DRAWINGS

Various other objects, features, and many of the attendant advantages ofthe present invention will be readily appreciated as the same becomesbetter understood with reference to the following detailed descriptionof the preferred embodiments when considered in connection with theaccompanying drawings, in which:

FIG. 1 is a sketch of an etching apparatus used for etching a Group IIInitride semiconductor according to embodiments;

FIG. 2 is a plan view showing a Group III nitride semiconductor to beetched using the method for etching a Group III nitride semiconductoraccording to Embodiment 1;

FIG. 3 shows an A-A cross-section of FIG. 2.

FIG. 4 is a cross-section of the Group III nitride semiconductor etchedby the method for etching a Group III nitride semiconductor according toEmbodiment 1;

FIG. 5 is a cross-section of the Group III nitride semiconductor etchedby the method for etching a Group III nitride semiconductor according toEmbodiment 1;

FIG. 6 is a plan view of a Group III nitride semiconductor to be etchedby a method for etching a Group III nitride semiconductor according tovariation of Embodiment 1.

FIG. 7 is a sketch of a semiconductor crystal production apparatusemployed in a method for producing a Group III nitride semiconductorcrystal according to Embodiment 2;

FIG. 8 is a sketch for describing a seed crystal preparation stepemployed in the method for producing a Group III nitride semiconductorcrystal according to Embodiment 2;

FIG. 9 is a sketch (part 1) for describing a seed crystal employed inthe method for producing a Group III nitride semiconductor crystalaccording to Embodiment 2;

FIG. 10 is a sketch (part 2) for describing a seed crystal employed inthe method for producing a Group III nitride semiconductor crystalaccording to Embodiment 2;

FIG. 11 is a sketch for describing the method for producing a Group IIInitride semiconductor crystal according to Embodiment 2;

FIG. 12 is a sketch (part 1) for describing a Group III nitridesemiconductor crystal according to Embodiment 2; and

FIG. 13 is a sketch (part 2) for describing a Group III nitridesemiconductor crystal according to Embodiment 2.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Hereinafter, specific embodiments of the present invention will bedescribed with reference to the drawings. However, these embodiments aregiven only for the purpose of illustration and should not be construedas limiting the invention thereto. In the drawings, the thickness ofeach layer is not an actually measured one but a conceptual thickness.

Embodiments of a method for etching a Group III nitride semiconductor, amethod for producing a Group III nitride semiconductor crystal using thesame and a method for producing a GaN substrate using the same will bedescribed. In Embodiment 1, a method for etching a Group III nitridesemiconductor is described. In Embodiments 2 and 3, a method forproducing a Group III nitride semiconductor crystal using the etchingmethod of Embodiment 1 is described. In Embodiment 4, a method forproducing a GaN substrate using the method for producing a Group IIInitride semiconductor crystal of Embodiments 2 and 3 is described.

Embodiment 1

Embodiment 1 will be described. In Embodiment 1, a method for etching aGroup III nitride semiconductor in a melt containing Na is described.

1. Etching Apparatus

FIG. 1 is a sketch of an etching apparatus 1 employed in a method foretching a Group III nitride semiconductor according to Embodiment 1. Theetching apparatus 1 is an apparatus for etching a Group III nitridesemiconductor V1. The etching apparatus 1 includes a chamber 2, a supplypipe 3, and a discharge pipe 4. The chamber 2 is a container that cancontain an etching solution. The chamber 2 can also contain a Group IIInitride semiconductor V1 to be etched and an etching solution 5.

The supply pipe 3 is to supply gas into the chamber 2. The dischargepipe 4 is to discharge gas from the chamber 2. Through operation ofvalves (not illustrated) attached to the supply pipe 3 and the dischargepipe 4, the inside of the chamber 2 is replaced with inert gas (e.g. N₂)or the pressure is controlled.

A semiconductor crystal production apparatus 10 (see FIG. 7) describedin Embodiment 2 may be used other than the etching apparatus 1 shown inFIG. 1. Other etching apparatus may be used as long as it can containthe Group III nitride semiconductor V1 and the etching solution 5 in thecontainer and adjust the temperature or pressure inside the container.

2. Group III Nitride Semiconductor (Etching Target)

FIG. 2 shows a plan view of a Group III nitride semiconductor V1 to beetched as viewed from the top. FIG. 3 is an A-A cross-section of FIG. 2.The Group III nitride semiconductor V1 comprises an underlayer U1, amask layer M1, and a protective film PF. A protective film PF is formedon a first surface of the underlayer U1 and a mask layer M1 is formed ona second surface of the underlayer U1. The underlayer U1 is made of GaN.The first surface of the underlayer U1 is a N polarity surface. That is,the protective film PF is formed on the N polarity surface of GaN. Thesecond surface of the underlayer U1 is a Ga polarity surface, i.e.,+c-plane (0001). The mask layer M1 is formed of a single crystal AlGaN.The mask layer M1 of AlGaN is epitaxially grown along the +c-axis ofAlGaN on the second surface of the underlayer U1 of GaN through MOCVDand is etched in a predetermined pattern as shown in FIG. 2 byphotolithography. Accordingly a top surface of AlGaN is a c-plane (Ga orAl polarity surface). The protective film PF is formed of Al₂O₃.

The mask layer M1 covers a portion of the second surface of theunderlayer U1. Therefore, the underlayer U1 has exposed portions E1which are not covered with the mask layer M1, and unexposed portions E2which are covered with the mask layer M1. As shown in FIG. 2, theexposed portions E1 are hexagon shaped. The exposed portions E1 aredisposed in a honeycomb structure. The side surface C1 of the mask layerM1 framing the exposed portions E1 is a surface perpendicular to am-axis of GaN and AlGaN. That is, the side surface C1 is a m-plane ofAlGaN.

Thus, a plurality of concave portions X1 is formed on the Group IIInitride semiconductor V1. The concave portions X1 are disposed in ahoneycomb structure and hexagon shaped. Each concave portion X1 has theexposed portion E1 of the underlayer.

The Al composition mol ratio (hereinafter, referred to as “compositionratio”) X of the mask layer M1 is preferably 0.02 to 1.0. Morepreferably, the Al composition ratio X of the mask layer M1 is 0.03 to0.50 as shown in Table 1. When the Al composition ratio X is less than0.02, the mask layer M1 is rapidly dissolved by the etching solution 5.The Al composition ratio of the underlayer U1 is preferably 0 to 0.02.When the Al composition ratio is larger than 0.02, the underlayer U1 isdifficult to etch. Needless to say, the Al composition ratio of theunderlayer U1 is smaller than that of the mask layer M1 to remove theunderlayer by etching.

Thus, a Group III nitride semiconductor V1 is prepared for etching ofEmbodiment 1. The Group III nitride semiconductor V1 has unexposedportions E2 of the underlayer U1 which are covered with the mask layerM1 and exposed portions E1, i.e., the remaining portion of theunderlayer U1 which is not covered with the mask layer M1. That is, theunderlayer U1 is partially covered with the mask layer M1.

TABLE 1 Al composition ratio of mask layer 0.03 to 0.50 Thickness ofmask layer 2 nm to 2 μm

3. Atomic Layer Deposition (ALD)

In Embodiment 1, a protective film PF is formed on a first surface ofunderlayer U1 through atomic layer deposition (ALD). ALD is superior inreproducibility of film quality and film thickness. The protective filmPF is formed of, for example, Al₂O₃. At this time, the protective filmPF is hardly dissolved even if subjected to etching. The thickness ofthe protective film PF is 10 nm to 200 nm. A well-known apparatus may beused in atomic layer deposition.

4. Etching Method

The etching method will next be described. In the etching method ofEmbodiment 1, a Group III nitride semiconductor V1 is etched in anetching solution 5. The Group III nitride semiconductor V1 and theetching solution 5 are placed in the chamber 2 of the above mentionedetching apparatus 1. Here, the etching solution 5 is a melt containingat least sodium (Na).

Table 2 shows the inside temperature and pressure conditions of thechamber 2 for etching. The melt may contain carbon (C) a little. Table 2also shows the carbon content. Evaporation of the melt can be preventedby applying pressure inside the chamber 2. The etching time is 0.5 hoursto 10 hour, which is only a guide. The gas supplied is Ar or N₂. Theetching temperature is 600° C. to 1,000° C.

In the etching method of Embodiment 1, the mask layer M1 and theprotective film PF are hardly dissolved and the underlayer U1 isdissolved. That is, the second surface of the underlayer U1, morespecifically, the exposed portions E1 which are not covered with themask layer M1, are etched. On the other hand, the AlGaN layer is hardlydissolved in the Na melt. However, actually, the mask layer M1 isslightly etched from the side surface C1. The amount of such sideetching is larger at a higher temperature. Therefore, the temperature ofthe Na melt is preferably 830° C. or less. Within this temperaturerange, the etching rate of the N polarity surface is larger than that ofthe Ga polarity surface. However, in Embodiment 1, only Ga polaritysurface can be etched since the protective film is formed on the Npolarity surface.

More preferably, the temperature of the Na melt is 700° C. to 830° C.When etching is performed within this temperature range, a smootherfacet plane is exposed. The etching rate of GaN increases by addingcarbon. However, the etching rate of the N polarity surface alsodrastically increases by adding carbon. In Embodiment 1, only the Gapolarity surface can be etched since the protective film is formed onthe N polarity surface. This is because the solubility of GaN in the Namelt containing a carbon content shown in Table 2 is higher than thesolubility of GaN in the Na melt containing no carbon.

TABLE 2 Temperature 600° C. or more 1,000° C. or less Temperature (morepreferable) 700° C. or more 830° C. or less Pressure 1 MPa or more 10MPa or less Carbon content 0 mol % or more 1 mol % or less

5. Group III Nitride Semiconductor after Etching

FIG. 4 is a sketch of Group III nitride semiconductor V2 etched by theetching method of Embodiment 1. As shown in FIG. 4, concave portions X2are formed on the Group III nitride semiconductor V2, which are deeperthan before etching. A facet plane can be exposed in the concaveportions X2. For example, the surface F1 is a {10-11} plane. The surfaceF2 is a {0001} plane. However, the Group III nitride semiconductor V2can be etched until the {0001} plane disappears.

When the facet plane is exposed in this way, the dissolution rate of theexposed facet plane is sufficiently slow. Therefore, once the facetplane is exposed during etching, it hardly disappears even if etchingcontinues after that. Usually once the facet plane is formed, theetching rate of the Ga polarity surface is reduced, and the N polaritysurface etching is remarkable. However, in Embodiment 1, the N polaritysurface is prevented from dissolving and the Ga polarity surface etchingis never stopped during the process since the protective film PF isformed on the N polarity surface. The etching depth can be controlled bythe width of opening and the etching time. For example, etching to adepth of about 300 μm is possible.

6. Variation 6-1. When the Mask Layer is not Formed

In Embodiment 1, the mask layer M1 is formed on the Group III nitridesemiconductor V1. However, even when the mask layer M1 is notnecessarily formed, etching is possible. For example, as shown in FIG.5, a Group III nitride semiconductor V4 (shown by the actual line ofFIG. 5) having concave portions X4 can be produced by etching a GroupIII nitride semiconductor V3 (shown by the dotted line of FIG. 5) havingconcave portions X3 and no mask layer. Here, a facet plane F3 can beexposed in the concave portion X4. The melt temperature at that time maybe the same as that shown in Table 2. Thus, the Group III nitridesemiconductor can be etched. FIG. 5 is only a sketch. In FIG. 5, theprotective film PF seems to be etched, but actually it is hardly etched.

6-2. Shape of Concave Portion of Group III Nitride Semiconductor

In Embodiment 1, in a plan view hexagonal concave portions X1 areformed. However, the concave portion X1 may be another polygon or acircle in a plan view. For example, as shown in FIG. 6, triangle concaveportions X5 may be formed. The concave portion X5 has a truncatedtriangular pyramid shape or a triangle prism shape. Each side length W51of the triangle is 10 μm to 2,000 μm. The width W52 of the mask layer540 is 10 μm to 200 μm. However, they are not limited to these values.

In the Group III nitride semiconductor V1 shown in FIG. 3, the secondsurface of the underlayer U1 is flat. That is, the exposed portions E1and the unexposed portions E2 are on the same flat surface. However, theexposed portions E1 may be further removed. This case will be explainedin Embodiment 2.

6-3. Etching Solution

In Embodiment 1, a melt containing Na is used as the etching solution 5.The etching solution 5 may contain Ga in addition to Na. In this case,the Ga content is preferably 5 mol % or less. When the Ga contentexceeds 5 mol %, N dissolution from the vapor phase increases and theetching rate is reduced. Therefore, even if GaN is dissolved from theunderlayer U1, the Ga content of the molten mixture must be 5 mol % orless. Thus, the Na amount of the etching solution is adjusted,considering the dissolution amount of Ga. The etching solution 5 maycontain other Group III nitride metal.

6-4. Material of Underlayer

In Embodiment 1, a GaN layer is formed as the underlayer U1. However, aGroup III nitride semiconductor having an Al composition ratio of 0 to0.02 may be formed as the underlayer U1. The smaller the Al compositionratio, the easier the dissolution of the underlayer by etching. The Alcomposition ratio of the underlayer U1 is smaller than that of the masklayer M1 to appropriately dissolve the underlayer U1.

6-5. Material of Protective Film

In Embodiment 1, the protective film PF is formed of Al₂O₃ throughatomic layer deposition (ALD). However, other material may be employedas long as it is not dissolved in the etching solution through etching.For example, silicon oxide, silicon nitride, titan oxide (TiO₂) orzirconium oxide (ZrO₂) may be employed. In addition, a metal having highmelting point may be employed.

7. Summary of Embodiment 1

As described above, a Group III nitride semiconductor V1 having anunderlayer U1, a mask layer M1, and a protective film PF is etched in aNa melt in the method for etching a Group III nitride semiconductor ofEmbodiment 1. Therefore, exposed portions E1 of the concave portions X1are sufficiently etched, to thereby form concave portions X2 in whichfacet planes are exposed. Moreover, this etching method enables theformation of concave portions having a depth of hundreds μm or more.

Notably, Embodiment 1 is given for the purpose of illustration only, andneedless to say, those skilled in the art can conceive variousmodifications and variations, so long as the scope of the invention isnot impaired. The number of concave portions provided in the Group IIInitride semiconductor V1 is actually a larger number, as compared withthe number of concave portions illustrated in the drawings.

Embodiment 2

Embodiment 2 will be described. Embodiment 2 is directed to a method forproducing a Group III nitride in which a Group III nitride semiconductorcrystal is grown on a GaN self-standing substrate using the method foretching a Group III nitride semiconductor described in Embodiment 1.

1. Semiconductor Crystal Production Apparatus

The configuration of the production apparatus for the GaN crystal ofEmbodiment 2 will be described. As shown in FIG. 7, the semiconductorcrystal production apparatus 10 includes a pressure container 20, areaction vessel 11, a crucible 12, a heating apparatus 13, supply pipes14, 16, and discharge pipes 15, 17.

The pressure container 20 is a pressure-resistant hollow cylinder madeof stainless steel. To the pressure container 20, the supply pipe 16 andthe discharge pipe 17 are connected. In the pressure container 20, thereaction vessel 11 and the heating apparatus 13 are disposed. Throughplacing the reaction vessel 11 in the pressure container 20, thereaction vessel 11 does not require particularly high pressureresistance. Thus, the reaction vessel 11 may be made of an inexpensivematerial, and recyclability is improved.

The reaction vessel 11 is made of SUS and has heat resistance. In thereaction vessel 11, the crucible 12 is placed. The crucible 12 is madeof a material such as W (tungsten), Mo (molybdenum), BN (boron nitride),alumina, or YAG (yttrium aluminum garnet). The crucible 12 holds amolten mixture 21 containing Ga and Na, and a seed crystal T10 ismaintained in the molten mixture 21.

To the reaction vessel 11, the supply pipe 14 and the discharge pipe 15are connected. Through operation of valves (not illustrated) attached tothe supply pipe 14 and the discharge pipe 15, there are performedaeration in and feeding nitrogen into the reaction vessel 11, andcontrolling the pressure inside the reaction vessel 11. Nitrogen is alsosupplied to the pressure container 20 via the supply pipe 16. Throughoperation of valves (not illustrated) attached to the supply pipe 16 andthe discharge pipe 17, the nitrogen flow rate and discharge rate arecontrolled, thereby the pressure inside the pressure container 20 isvirtually equalized with that of the reaction vessel 11. The temperatureinside the reaction vessel 11 is controlled by means of the heatingapparatus 13.

There is provided an apparatus which can rotate the crucible 12 so as tostir the molten mixture 21 contained in the crucible 12, thereby themolten mixture 21 is stirred during the growth of a GaN crystal. Byvirtue of the apparatus, the molten mixture 21 can have a uniform Na,Ga, or N concentration distribution profile, thereby a GaN crystal ofuniform quality can be grown. The apparatus which can rotate thecrucible 12 has a rotation axis 22, a table 23, and a driving unit 24.The rotation axis 22 extends from the inside of the reaction vessel 11to the outside of the pressure container 20. The table 23 is disposed inthe reaction vessel 11 and is connected to the rotation axis 22 so thatit sustains the crucible 12. The driving unit 24 controls rotation ofthe rotation axis 22. The table 23 is rotated through rotation of therotation axis 22 driven by the driving unit 24, thereby the crucible 12sustained by the table 23 is rotated.

Meanwhile, when the employed reaction vessel 11 has pressure resistance,the pressure container 20 is not necessarily employed. In addition, inorder to prevent vaporization of Na during growth of a GaN crystal, thecrucible 12 may be provided with a lid. Instead of or in addition to thecrucible 12 rotating means, crucible 12 swinging means may be provided.

2. Method for Producing a Group III Nitride Semiconductor Crystal

The method of Embodiment 2 for producing a Group III nitridesemiconductor crystal includes the following steps:

(A) Seed crystal preparation step,

-   -   (A-1) Mask layer growth step,    -   (A-2) Protective film formation step,    -   (A-3) Concave portion formation step,

(B) Seed crystal etching step, and

(C) Semiconductor crystal formation step.

These steps will next be described in detail.

2-1. (A) Seed Crystal Preparation Step 2-1-1. (A-1) Mask Layer GrowthStep

Firstly, a GaN substrate G10 is provided. The GaN substrate G10 is aself-standing GaN substrate and has a dislocation density of about5×10⁶/cm². The GaN substrate G10 also serves as an underlayer on which amask layer is to be formed. Thus, a mask layer 140 is formed on the GaNsubstrate G10. Eventually, a stacked body B11 shown in FIG. 8 isproduced.

The mask layer 140 has a composition of Al_(X)In_(Y)Ga_((1-X-Y))N (0≦X,0≦Y, X+Y≦1). The mask layer 140 is preferably an AlGaN layer. The Alcomposition ratio X of the mask layer 140 is preferably 0.02 to 1.0.Particularly, the Al composition ratio X of the mask layer 140 is morepreferably 0.03 to 0.50, as shown in Table 1. When the Al compositionratio X is less than 0.03, the mask layer is readily melted back by aflux, whereas when the Al composition ratio X is more than 0.50, thequality of the GaN crystal formed in the below-described semiconductorcrystal formation step may be adversely affected.

As shown in Table 1, the mask layer 140 preferably has a thickness of 2nm to 2 μm. When the thickness of the mask layer 140 is less than 2 nm,the effect of melting back is not sufficient, whereas when the thicknessof the mask layer 140 is in excess of 2 μm, the quality of the GaNcrystal formed in the below-described semiconductor crystal formationstep is impaired. In this mask layer growth step, the entire secondsurface of the underlayer U1 is uniformly covered with the mask layer140. The mask layer 140 is formed by epitaxially growing AlInGaN on theGaN substrate G10 through MOCVD.

2-1-2. (A-2) Protective Film Formation Step

In Embodiment 2, a protective film PF is formed on the stacked body B11through atomic layer deposition (ALD). The apparatus and method employedin Embodiment 2, are the same as in Embodiment 1. However, Embodiment 2has a semiconductor crystal formation step as a latter step in additionto the etching step. Therefore, in the semiconductor crystal formationstep, the protective film PF must not be dissolved. The materials listedin Embodiment 1 and Variation of Embodiment 1 may be used as thematerial of the protective film PF. By virtue of this step, theprotective film PF is formed on the first surface of the stacked bodyB11, and eventually a stacked body B12 is produced.

2-1-3. (A-3) Concave Portion Formation Step 2-1-3-1. Concave PortionFormation Procedure

Then, a plurality of concave portions X11 is formed in the stacked bodyB11 having the protective film PF on a back surface thereof, which isdefined as a stacked body B12 shown in FIG. 9. FIG. 10 shows a B-Bcross-section of FIG. 9. As shown in FIG. 10, a plurality of concaveportions X11 are formed by removing an area of the mask layer 140through the full thickness and the corresponding area of the GaNsubstrate G10 through a partial thickness. In each concave portion X11,the GaN substrate G10 is exposed. Through the above procedure, as shownin FIGS. 9 and 10, a seed crystal T10 having a plurality of concaveportions X11 is produced. The concave portions X11 may be formedthrough, for example, photolithography. Firstly, patterning of resist isperformed. Then, an area of the mask layer 140 through the fullthickness and the corresponding area of the GaN substrate G10 through apartial thickness are removed through dry etching, to thereby form aplurality of concave portions X11. After formation of the concaveportions X11, the mask layer 140 serves as a mask portion which coversthe underlayer. Subsequently, the resist mask is removed, thereby theseed crystal T10 shown in FIGS. 9 and 10 is produced. Then, the seedcrystal T10 shown in FIGS. 9 and 10 is produced. Then, the seed crystalT10 provided with a plurality of a plurality of concave portions X11 iswashed.

2-1-3-2. Seed Crystal Provided with Concave Portions

The seed crystal T10 includes a GaN substrate G10, a protective film PF,and a mask layer 140. While the protective film PF is formed on thefirst surface of the GaN substrate G10, the mask layer 140 is formed onthe second surface of the GaN substrate G10. As shown in FIG. 9, theconcave portions X11 are disposed in a honeycomb structure on thesurface 142 of the mask layer 140. As viewed from the top of the masklayer 140, each concave portion X11 of the seed crystal T10 has ahexagon shape.

As shown in FIG. 10, each concave portion X11 is a non-through holewhich passes through the mask layer 140 through the full thickness andremoves the corresponding area of the GaN substrate G10 through apartial thickness. While the mask layer 140 has a thickness of 2 nm to 2μm, each concave portion X11 has a depth D1 which is larger than thethickness of the mask layer 140. The depth D1 of the concave portion X11is 1 μm to 5 μm. In the case of formation of the concave portions X11,one requirement is that a part of the GaN substrate G10 (i.e., GaNlayer) is exposed to the bottom surface of each concave portion X11.

In each concave portion X11, each side of hexagon W0 (see FIG. 9) has alength of 10 μm to 2,000 μm. When the side length W0 is shorter than 10μm, the effect of dislocation reduction is not sufficiently obtained.

The spacing between two adjacent concave portions X11 (W2) is 10 μm to200 μm. The spacing W2 is less than 10 μm, the area of the surfaces 142,which serve as starting points of lateral growth of a semiconductorlayers grown in the below-described semiconductor crystal formationstep, is difficult to secure.

Each concave portion X11 has a bottom surface G12 a and side surfacesG11 a, 141. The bottom surface G12 a is a part of the GaN substrate G10.The side surfaces G11 a, 141 are generally orthogonal to the surface 142of the mask layer 140. The side surfaces G11 a, 141 are formed to passthrough the mask layer 140 through the full thickness and the GaNsubstrate G10 through a partial thickness. As shown in FIG. 10, the seedcrystal T10 has an unexposed portion E4 of the GaN substrate G10, whichis covered with the mask layer 140, and an exposed portion E3, i.e., theremaining portion of the GaN substrate G10, which is not covered withthe mask layer 140.

2-2. (B) Seed Crystal Etching Step

Then, the seed crystal T10 is etched through the etching methoddescribed in Embodiment 1. That is, the seed crystal T10 and the etchingsolution 5 are placed in the chamber 2 of the etching apparatus 1. Underthe conditions shown in Table 2, etching is performed. The etchingsolution 5 has a Ga concentration of 5 mol %.

During the above step, portions of the GaN substrate G10, which havebeen provided to serve as side surfaces of the concave portions X11, aredissolved in the etching solution 5. Specifically, the bottom surfacesG12 a and the side surfaces G11 a are dissolved in the etching solution5. On the contrary, the mask layer 140 is difficult to dissolve in theetching solution 5. However, since the GaN substrate G10, serving as anunderlayer with respect to the mask layer 140, is dissolved, the masklayer 140 is slowly dissolved on the lateral side. Thus, the dimensionsof each concave portion X11 increase. More specifically, the depth ofthe concave portion X11 increases, and the width thereof increasesslightly.

The etching of GaN substrate G10 does not occur on the first surfaceside of the GaN substrate G10, i.e., the protective film PF side, sincethe GaN substrate G10 is not exposed on the protective film PF side. Theexposed portions E3 are subjected to etching on the second surface sideof the GaN substrate G10, i.e., the mask layer 140 side. Thus, the seedcrystal T10 is etched, and the facet plane of the GaN substrate G10 isformed, to thereby yield a seed crystal T11 shown in FIG. 11.

The concave portions X11 are subjected to etching, to provide concaveportions X12 shown in FIG. 11. Each concave portion X12 has inclinedplanes G13, a bottom surface G14, and side surfaces 143. The inclinedplane G13 corresponds to a (10-11) plane. The bottom surface G14corresponds to a {0001} plane. Thus, facet planes are exposed in theconcave portion X12. However, the inclined plane G13 is not necessarilya complete {10-11} plane. It may be a collective surface of small{10-11} planes, i.e., terraces of {10-11}. Similarly, the bottom surfaceG14 may not be necessarily a complete {0001} plane.

2-3. (C) Semiconductor Crystal Formation Step

Subsequently, a semiconductor crystal layer is formed on the seedcrystal T11 through a flux method, which is a technique of liquid phaseepitaxy. That is, the seed crystal T11 and the raw materials are placedin the semiconductor production apparatus 10. Table 3 shows the rawmaterials (flux) employed herein. The Ga ratio is preferably 5 mol % to40 mol %. The carbon ratio may be varied from 0 mol % to 2.0 mol %. Thatis, the flux may or may not contain carbon, and preferably has a carboncontent of 0.01 mol % to 2.0 mol %. Notably, the amounts of the elementsshown in Table 3 are merely examples, and other amounts may be employed.

Needless to say, the target semiconductor crystal is a Group III nitridesemiconductor crystal, which may be GaN, AlGaN, InGaN, AlInGaN, etc.Firstly, the raw materials shown in Table 3 are weighed in a glovebox inwhich dew point and oxygen level are controlled. Notably, the amounts ofthe raw materials shown in Table 3 are merely examples, and otheramounts may be employed. Then, the seed crystal T11 and the rawmaterials are placed in a crucible 12 made of alumina. Next, thecrucible 12 is placed on a turn-table 23 disposed in the reaction vessel11 in the semiconductor crystal production apparatus 10. The pressurecontainer 20 is evacuated, and the pressure and temperature inside thecontainer 20 are elevated. At this time, nitrogen gas as one of the rawmaterials is supplied inside the reaction vessel 11.

TABLE 3 Ga 20 g to 80 g Na 20 g to 80 g C 0.01 mol % to 2.0 mol % (basedon Na)

Table 4 shows the conditions employed in the above step and in thecrucible. Actually, the temperature is 870° C., and the pressure is 4MPa. Under these conditions, the aforementioned materials are melted toform a molten mixture. The mixture is stirred at 20 rpm. The directionof the rotation of the turn-table 23 is appropriately altered in apredetermined period. At this time, melting back of the seed crystal T11hardly occurs since the facet planes have been exposed in the seedcrystal T11, and the seed crystal T11 is difficult to dissolve in themolten mixture. Thereby, a semiconductor crystal is grown on the seedcrystal T11. The growth time is 30 hours.

TABLE 4 Temperature 850° C. to 1,000° C. Pressure 3 MPa to 10 MPaStirring condition 0 rpm to 100 rpm Growth time 20 to 200 hours

After the flux has been saturated through pressurization, a GaN layer150 is grown on the seed crystal T11 in the molten mixture. At thistime, the GaN layer 150 grows on the mask layer 140. That is, the GaNlayer 150 grows from the surface 144 of the mask layer 140 in the upward(vertical) direction and the lateral direction over the concave portionsX12. On the other hand, GaN hardly grows on the inclined plane G13, thebottom surface G14, the side surface 143. The reason for this is thatthe growth rate of the semiconductor layer on the facet plane isextremely slow, and nitrogen (N) is difficult to supply to the concaveportion X12. Therefore, as shown in FIG. 12, the GaN layer 150 is grownsuch that the facet plane of the concave portion X12 is not buried withthe Group III nitride semiconductor.

3. Produced Group III Nitride Semiconductor Crystal 3-1. GaN Crystal

As described above, a GaN crystal C10 as shown in FIG. 12 is producedthrough the method for producing a Group III nitride semiconductorcrystal of Embodiment 2. The GaN crystal C10 has the GaN substrate G11,the mask layer 140, the GaN layer 150, non-crystal portions X13, and theprotective film PF. The GaN layer 150 is a GaN single crystal.

Non-crystal portions X13 are portions in which no semiconductor crystalhas been formed. Each non-crystal portion X13 assumes a space. However,actually, the space is filed with a flux. Each non-crystal portion X13is defined by the inclined planes G13 (110-111 plane) of the GaNsubstrate G11 and a portion 152 of the bottom surface 151 of the GaNlayer 150.

3-2. Shape of Crystal

The bottom surface 151 of the GaN layer 150 is in contact with the masklayer 140 and the non-crystal portions X13. Portions 152 of the bottomsurface 151 of the GaN layer 150 are in contact with the non-crystalportions X13. Each of the portions 152 of the bottom surface 151 of theGaN layer 150 which is in contact with the non-crystal portion X13generally assumes the form of a hexagon as viewed from the top. Theremaining portions 153 of the bottom surface 151 of the GaN layer 150are in contact with the mask layer 140. The bottom surface 151 of theGaN layer 150 is flat with a step of a thickness of the mask layer 140.As described in the Examples hereinbelow, the thickness of the GaN layer150 may be adjusted to about 1 mm.

3-3. Dislocation Density of Crystal

The GaN crystal B12 of Embodiment 2 has non-crystal portions X13.Therefore, during the growth of the GaN layer 150 from the GaN substrateG10, dislocations do not extend from the portions 152 of the bottomsurface 151 of the GaN layer 150. In other words, some dislocations arenot inherited from the underlayer by the GaN substrate. However,dislocations are inherited from the mask layer 140. Thus, sinceinheritance of dislocations from the underlayer is partially inhibited,the GaN layer 150 has excellent crystallinity. Specifically, the GaNlayer 150 has an average dislocation density of the whole surface of1×10⁵/cm² or less.

3-4. Separability of Crystal

Regarding the GaN crystal B12 of Embodiment 2, the GaN layer 150 can bereadily separated from the GaN substrate G10, since the stressattributed to warpage of the seed crystal or the like is applied mainlyto the interface between the seed crystal T11 and the GaN layer 150. Insome cases, the seed crystal is spontaneously removed from the crystalat the time of temperature lowering performed during crystal growth.Alternatively, by applying slight impact to the stacked body aftercrystal growth, the seed crystal may be removed from the crystal. FIG.13 shows the GaN layer 150 and the seed crystal T11 after separation.Thus, the GaN layer 150 is readily removed from the GaN substrate G11,by virtue of non-crystal portions X13 provided between the growthsubstrate and the GaN layer 150.

As described above, concave portions X11 and non-crystal portions X13are intentionally provided in order to intercept inheritance ofdislocations, thereby a Group III nitride semiconductor crystal whichhas excellent crystallinity and which can be readily separated from thegrowth substrate can be produced.

4. Variation 4-1. Group III Nitride Semiconductor Crystal

In Embodiment 2, a GaN layer 150 is formed. However, the method of theinvention may be applied to production of other Group III nitridesemiconductor crystals than GaN. That is, the production method of theinvention is applicable to production of Al_(X)In_(Y)Ga_((1-X-Y))N (0≦X,0≦Y, X+Y≦1).

4-2. Seed Crystal Etching Step and Semiconductor Crystal Formation Step

In Embodiment 2, the seed crystal etching step and the semiconductorcrystal formation step are performed separately using a differentapparatus. Actually, the seed crystal etching step and the semiconductorcrystal formation step may be sequentially performed in thesemiconductor crystal production apparatus 10. In that case, thesesequential steps can be performed by using the etching solution 5containing Ga or adding only Ga to the etching solution 5 after the seedcrystal etching step.

4-3. Seed Crystal Preparation Procedure

In Embodiment 2, as the seed crystal preparation step, the mask layergrowth step, the protective film formation step, and the concave portionformation step are performed in this order. However, these steps may beperformed in the rearranged order. In this case, the concave portionformation step is performed after the mask layer growth step. Therefore,for example, the mask layer growth step, the concave formation step, andthe protective film formation step may be performed in this order.

4-4. Shape of Concave Portion

In Embodiment 2, the seed crystal T10 having hexagonal concave portionsX11 is employed. The plane shape of the concave portion X11 may beanother polygon such as hexagon, or a circle. The concave portionpreferably has a plane for which a facet plane of the GaN substrate G10is easy to expose through Na etching.

4-5. Material of Protective Film

In Embodiment 2, the protective film PF is formed of Al₂O₃ throughatomic layer deposition (ALD). However, other material may be employedas long as it is not dissolved in the etching solution through etching.

Here, when a silicon compound is employed as the material of theprotective film PF, the silicon compound is dissolved in flux, tothereby inhibit the growth of GaN crystal. Therefore, a materialcontaining Si such as SiO₂ is not preferable. However, if SiO₂ isremoved after the seed crystal etching step, GaN crystal can be grown.

5. Summary of Embodiment 2

As described above, a seed crystal having the protective film PF on thefirst surface and the mask layer 140 on the second surface is used asthe seed crystal T10 for the flux method in the method for producing aGroup III nitride semiconductor crystal according to Embodiment 2. Thesecond surface is provided with the concave portions X11. Asemiconductor crystal is grown on the mask layer 140. Therefore, nosemiconductor crystal is formed in each concave portion X12, andinstead, the concave portion X12 is provided with a non-crystal portionX13. That is, no dislocations of the semiconductor layer below thenon-crystal portions X13 are transferred to the GaN layer 150 disposedon the non-crystal portions X13. In other words, the thus-formed GaNcrystal has satisfactorily low dislocation density. Thus, a Group IIInitride semiconductor crystal having excellent crystallinity can beformed.

Notably, Embodiment 2 is given for the purpose of illustration only, andneedless to say, those skilled in the art can conceive variousmodifications and variations, so long as the scope of the invention isnot impaired. The number of concave portions provided in the seedcrystal is actually a larger number, as compared with the number ofconcave portions illustrated in the drawings.

Embodiment 3

Embodiment 3 will be described. In Embodiment 3, different fromEmbodiment 2, the seed crystal etching step by Na molten of Embodiment 2is not performed.

1. Method for Producing a Group III Nitride Semiconductor Crystal

The method of Embodiment 3 for producing a Group III nitridesemiconductor crystal includes the following steps:

(A) Seed crystal preparation step,

-   -   (A-1) Mask layer growth step,    -   (A-2) Protective film formation step,    -   (A-3) Concave portion formation step, and

(D) Semiconductor crystal formations step,

(D) Semiconductor crystal formations step is slightly different fromthat of Embodiment 2. Therefore, the differences are mainly describedbelow.

1-1. (D) Semiconductor Crystal Formations Step

In (D) Semiconductor crystal formations step, the seed crystal T10 isstill as shown in FIG. 10. A semiconductor crystal is grown using thematerials shown in Table 3 under the conditions shown in Table 4. Whenthe temperature and the pressure are increased, the exposed portions E3of the seed crystal T10 undergo melting back. At this time, the firstsurface of the seed crystal T10 has the protective film PF and thesecond surface of the seed crystal T10 has the mask layer 140. The sidesurfaces of the seed crystal T10 are very thin. Therefore, portionswhich mainly undergo melting back are the exposed portions E3. As aresult, the seed crystal T10 is etched, to thereby yield a seed crystalT11 shown in FIG. 11.

After the flux has been saturated in the semiconductor crystalproduction apparatus 10, a GaN layer 150 starts to grow from the masklayer 140 as a growth starting point, to thereby produce a GaN crystal510. This is the same as Embodiment 2.

2. Summary of Embodiment 3

In Embodiment 3, even if (B) Seed crystal etching step is not performed,the exposed portions E3 of the seed crystal T10 are subjected to etchingwhen the temperature and pressure are increased in (D) Semiconductorcrystal formation step. However, the thus-formed concave portions X12assume smaller than those in Embodiment 2. Therefore, the pressure andthe temperature are preferably adjusted so as to fully cause meltingback (etching).

Embodiment 4

Embodiment 4 will be described. In Embodiment 4, a method for producinga GaN substrate using the method for producing a Group III nitridesemiconductor crystal described in Embodiments 2 and 3.

1. Semiconductor Crystal Separation Step

As described above, in the GaN crystal C10 provided with non-crystalportions X13, the GaN layer 150 is readily removed from the growthsubstrate, since the presence of non-crystal portions X13 reducesadhesion strength to the underlayer or the internal stress is applied tothe interface. Therefore, as shown in FIG. 13, the GaN layer 150 isseparated from the seed crystal T11. That is, the GaN layer 150 of theGaN crystal C10 is removed from the seed crystal T11. Separation can bemanually done by an operator. Separation may be performed throughheating/cooling on the basis of the difference in thermal expansioncoefficient. The thus-separated crystal exhibits reduced warpage andhigh quality.

Actually, in some cases, the mask layer 140 and the non-crystal portionsX13 are partially adhered to the GaN crystal. In such a case, the bottomsurface 151 is ground, to thereby solve the problem.

2. Summary of Embodiment 4

As described above, the method for producing a GaN substrate ofEmbodiment 4, includes removing the GaN crystal formed in any ofEmbodiments 2 or 3 from the growth substrate, to thereby provide a GaNself-standing substrate. After the separation step, a step such asgrinding may be performed.

Example 1. Example 1 1-1. Seed Crystal

Example 1 will be described. In Example 1, similar to Embodiment 2, ac-plane GaN self-standing substrate was employed. The GaN self-standingsubstrate had a diameter of 2 inches (50.8 mm). After an i-GaN layer wasgrown so as to have a thickness of 1 μm on the second surface of the GaNself-standing substrate, an AlGaN layer having a thickness of 100 nm wasgrown. The AlGaN layer had an Al composition ratio of 0.1. The i-GaNlayer and the AlGaN layer are epitaxially grown on the GaN self-standingsubstrate through MOCVD.

Subsequently, through atomic layer deposition, an Al₂O₃ film was formedon the first surface so as to have a thickness of 100 nm.

Then, patterning of resist was performed through photolithography. Then,the AlGaN layer and the i-GaN layer were etched by ICP, to thereby aplurality of concave portions. The plane shape of the concave portionwas a hexagon. Each side of hexagon had a length of 300 μm. The masklayer had a width of 50 μm.

1-2. Etching

Next, Na (30 g), and C (80 mg) were placed in the etching apparatus. Atthis time, the carbon proportion of the flux was 0.5 mol %. After theapparatus was evacuated, the inside temperature and pressure wereincreased. Etching was performed at a temperature of 750° C. and apressure of 3 MPa. The mixture was stirred at 20 rpm while appropriatelyaltering the direction of the rotation. The etching was performed fortwo hours. The gas supplied inside the etching apparatus was nitrogen.Thus, the etched seed crystal was produced.

1-3. Growth

The seed crystal was placed in the semiconductor crystal productionapparatus. The raw materials of Ga (30 g), Na (30 g), and C (80 mg) wereplaced in the semiconductor crystal production apparatus. Then, whilesupplying nitrogen gas, the inside temperature and pressure wereincreased. The temperature was 870° C., and the pressure was 3 MPainside the apparatus. The mixture was stirred at 20 rpm whileappropriately altering the direction of the rotation. The growth time ofGaN crystal was 30 hours.

1-4. Result

As a result, a GaN crystal was produced. Portions corresponding tonon-crystal portions X13 were confirmed to be formed. Therefore, thegrown GaN layer could be readily separated from the seed crystal. Thegrown GaN layer had a thickness of 0.9 mm. The thus-obtained crystal hadan average dislocation density of the whole surface of 1×10⁵/cm² orless.

NOTES

1. In the etching step, a melt containing at least Na has a temperatureof 700° C. to 830° C.2. In the concave formation step, concave portions are formed so as tohave a hexagon shape and disposed in a honeycomb structure.3. In the etching step, a {10-11} plane of the underlayer is exposed.4. The protective film is formed through atomic layer deposition.5. The etching step comprises a mask layer formation step of forming amask layer made of Al_(X)In_(Y)Ga_((1-X-Y))N (0≦X, 0≦Y, X+Y≦1) on aportion of the second surface of the Group III nitride semiconductor, tothereby form unexposed portions of the second surface, which are coveredwith the mask layer and the remaining portion of the second surface,which is not covered with the mask layer, and in the etching step, theexposed portions of the second surface of the Group III nitridesemiconductor are etched after the mask layer formation step.6. The mask layer formation step further comprises a mask layer growthstep of growing a mask layer so as to uniformly cover the entire secondsurface of the underlayer made of Group III nitride semiconductor, and aconcave portion formation step of forming a plurality of concaveportions in the underlayer by removing an area of the mask layer throughthe full thickness and the corresponding area of the underlayer througha partial thickness.

What is claimed is:
 1. A method for etching a Group III nitridesemiconductor, the method comprising: forming a protective film on afirst surface being a N polarity surface of the Group III nitridesemiconductor; and etching at least a portion of a second surface of theGroup III nitride semiconductor in a melt containing at least Na.
 2. Themethod for etching a Group III nitride semiconductor according to claim1, wherein the method further comprising forming a mask layer made ofAl_(X)In_(Y)Ga_((1-X-Y))N (0≦X, 0≦Y, X+Y≦1) on a portion of the secondsurface of the Group III nitride semiconductor; and in etching a GroupIII nitride semiconductor, the second surface of the Group III nitridesemiconductor is etched after the mask layer was formed.
 3. The methodfor etching a Group III nitride semiconductor according to claim 2,wherein the mask layer is formed of AlGaN.
 4. The method for etching aGroup III nitride semiconductor according to claim 3, wherein theprotective film is formed of at least one selected from a groupconsisting of Al₂O₃, ZrO₂, and TiO₂.
 5. The method for etching a GroupIII nitride semiconductor according to claim 4, wherein the temperatureof the melt containing at least Na is 600° C. to 1,000° C.
 6. The methodfor etching a Group III nitride semiconductor according to claim 1,wherein the concentration of Group III metal in the melt containing atleast Na is 0 mol % to 5 mol %.
 7. A method for producing a Group IIInitride semiconductor crystal, the method comprising: forming aprotective film on a first surface being a N polarity surface of GroupIII nitride semiconductor; etching at least a portion of a secondsurface of the Group III nitride semiconductor in a melt containing atleast Na, to thereby form a seed crystal; and growing a target layer ofGroup III nitride semiconductor crystal on the seed crystal in a moltenmixture containing at least Group III metal and Na.
 8. The method forproducing a Group III nitride semiconductor crystal according to claim7, the method further comprising: forming a mask layer made ofAl_(X)In_(Y)Ga_((1-X-Y))N (0≦X, 0≦Y, X+Y≦1) on a portion of the secondsurface of the Group III nitride semiconductor, to thereby form anunexposed portion of the second surface which is covered with the masklayer and the remaining portion being an exposed portion of the secondsurface which is not covered with the mask layer; and etching theexposed portion of the second surface of the Group III nitridesemiconductor after the mask layer was formed.
 9. The method forproducing a Group III nitride semiconductor crystal according to claim8, wherein the forming a mask layer comprises: growing an uniform masklayer so as to cover the entire second surface of an underlayer made ofthe Group III nitride semiconductor; and forming a plurality of concaveportions in the underlayer by removing an area of the uniform mask layerto expose a part of the underlayer.
 10. The method for producing a GroupIII nitride semiconductor crystal according to claim 9, wherein, informing a mask layer, the mask layer is formed of AlGaN.
 11. The methodfor producing a Group III nitride semiconductor crystal according toclaim 8, wherein, in forming a protective film, the protective film isformed of at least one selected from a group consisting of Al₂O₃, ZrO₂,and TiO₂.
 12. The method for producing a Group III nitride semiconductorcrystal according to claim 9, wherein, in etching a Group III nitridesemiconductor, a facet plane of the underlayer is exposed.
 13. Themethod for producing a Group III nitride semiconductor crystal accordingto claim 12, wherein, in forming the target layer, the Group III nitridesemiconductor crystal is grown on the mask layer such that the facetplane of the underlayer is not buried with the Group III nitridesemiconductor crystal.
 14. The method for producing a Group III nitridesemiconductor crystal according to claim 13, wherein, in forming thetarget layer, a non-crystal portion defined by the facet plane and thebottom surface of the target layer is formed.
 15. The method forproducing a Group III nitride semiconductor crystal according to claim8, wherein the mask layer is formed of a Group III nitride semiconductorhaving an Al composition ratio of 0.02 to 1.00.
 16. The method forproducing a Group III nitride semiconductor crystal according to claim9, wherein the underlayer has an Al composition ratio of 0 to 0.02; andthe Al composition ratio of the underlayer is smaller than that of themask layer.
 17. The method for producing a Group III nitridesemiconductor crystal according to claim 9, wherein, in etching theunderlayer, the temperature of the melt containing at least Na is 600°C. to 1,000° C.
 18. A method for producing a GaN substrate, the methodcomprising: forming a protective film on a first surface being a Npolarity surface of Group III nitride semiconductor; etching at least aportion of a second surface of the Group III nitride semiconductor in amelt containing at least Na, to thereby form a seed crystal; growing aGaN crystal on the seed crystal in a molten mixture containing at leastGa and Na; and separating the GaN crystal from the seed crystal.